1. Field of the Invention
This invention relates in general to novel microporous fluorocarbon membranes and fluorocarbon filter cartridges using such membrane. More particularly, this invention relates to such novel membranes and cartridges used for filtration of particles sized from about .0.001.mu.m to about 1.0.mu.m. Removal of contaminants of these sizes using membranes and cartridges formed from fluorocarbons are often required in the electronics and pharmaceutical industries.
2. Description of the Prior Art
In the art of membrane filtration, it is well known that filter membranes can be categorized by the particle size of the contaminants which the filter devices are suited for removing. Of course, filter membranes suitable for removing smaller particles are more difficult to construct than those suitable for removing larger particles. In order to be suitable for removing a smaller size particle, a filter membrane must have pores which are slightly smaller than the particle to be removed and essentially no pores larger than the particle to be removed. The pores must have this uniformed size. It is also necessary to have a sufficiently large number of pores to provide a desired flux or flow rate per unit area of membrane.
As the particle pore size is reduced to achieve finer filtration, it is more and more difficult to achieve the desired uniform pore-size as well as a desired flux. This problem is accentuated when the pore size is less than 1.0.mu.m. At this size, it is especially difficult to achieve a desired uniform pore-size without having oversized pores. Moreover, in order to have sufficient flux, it is necessary to have a very thin membrane. With pores smaller than 1.0.mu.m, fluid friction caused by the thickness of the membrane is a substantial factor in reducing flux. Thus, not only must the membrane have a very high density of uniformly sized, very small pores, the membrane must be very thin. Of course, the thinner the membrane, the more fragile it is and the more difficult it is to prevent tearing, punctures or bursts of the membrane. A thin membrane is less able to withstand increased pressures which might otherwise be used to increase flux.
These problems are compounded when it is necessary to use a filter constructed of fluorocarbon, such as polytetrafluoroethylene. Fluorocarbon is often the only material for certain applications because it can be used at high temperatures, is chemically and physically resistant and it is chemically and physically inert. Because of these very properties it is difficult to construct a microporous (&lt;1.0.mu.m) fluorocarbon filter membrane and it is difficult to construct a fluorocarbon filter cartridge which uses such a membrane.
Porous fluorocarbon membranes or films, particularly ones fabricated in whole or in part from polytetrafluoroethylene, have been known for twenty five years or more. Such membranes or films are usually prepared by either of two general methods:
1. Incorporating a particulate, solid, pore forming filler removable by leaching, heating, etc. into a mass of powdered fluorocarbon polymer; shaping the resulting mixture into a preform, calendering the thus-obtained preform to reduce its thickness and form it into a self-sustaining sheet or film, with or without doubling over or cross-lapping the sheet or film between passes through the calender rolls; subjecting the calendered sheet or film to heat to dry the sheet or film and sinter the fluorocarbon polymer particles, and removing the particulate filler; see, for example, U.S. Pat. Nos. 3,281,511 to Goldsmith; 3,518,332 to Sklarchuk et al; 3,556,161 to Roberts; 3,661,645 to Strier et al; 3,773,878 to Jahnke; 3,859,402 to Bintliff et al; 3,864,124 to Breton et al; 3,890,417 to Vallance; 4,003,818 to Juillard et al; 4,196,070 to Chao et al; 4,250,002; 4,297,196 and 4,498,961 to Lazarz et al; 4,292,146 and 4,342,636 to Chang et al and 4,380,521 to Moreno et al, and British Patents Nos. 943,624 to Doulton & Co. Limited and 1,081,046 to Imperial Chemical Industries Limited.
2. Forming a porous fluorocarbon polymer membrane or film having nodes interconnected by fibrils using processes which involve, for example, stretching a preformed fluorocarbon polymer sheet, or extensively mixing and then biaxially calendering a putty-like mass containing fluorocarbon polymer; see, for example, U.S. Pat. Nos. 3,953,566 and 4,187,390 to Gore; 4,049,589 to Sakane; 4,089,758 to McAloon and 4,153,661 to Ree et al.
Commercially available porous polyfluorocarbon membranes or films made by the first of these methods, incorporation and subsequent removal of a particulate solid, are mostly useful only for coarse filtration, i.e., removal from liquids of particulate matter having average particle diameters of not less than about 1.0.mu.m, and in general also have inadequate strength, flux and dirt holding capacity to be useful in microfiltration applications. Hence, such membranes are used as coarse filters, or as separators or diaphragms in batteries and electrolytic cells.
Although porous polyfluorocarbon membranes or films having polymer nodes interconnected by fibrils have been used for microfiltration, such membranes are not without problems, some of which are attributable to their method of preparation. The membrane can itself be a major source of contamination because of its loosely bound fibrils and nodes. Also, it is difficult to control the processes by which such membranes are typically fabricated so that pore size will be correlated consistently with performance requirements.
An improved variation of the first general method outlined above and an improved membrane are revealed in U.S. Pat. No. 4,863,604 to Lo et al. In this patent an asymmetric microporous membrane is formed using two or more self-sustaining calendered sheets or films of fluorocarbon powder mixed with a leachable pore forming filler. Each sheet has a different average pore diameter so that a gradient is formed from one surface of the composite to the other. The composite is then laminated into an integral membrane by application of heat and the filler is removed to provide an asymmetric polyfluorocarbon membrane.
It is well known that the overall thickness of a filtration membrane is critical to an optimum flux. In other words, in order for a desirable high flow rate through the filtration membrane to be achieved, the thickness of the membrane must be minimized. However, in order to maintain the strength of the membrane, it is necessary that the thickness of the membrane be increased. Prior to this invention, the asymmetric membrane of the Lo et al patent was the best solution to this problem. However, it is still desirable to provide even further advantages with respect to strength and flux.
There is thus a need for chemically and physiologically inert microporous fluorocarbon membranes whose pore structures render them particularly suitable for microfiltration applications. There is also a need for improved fluorocarbon filter cartridges suitable for microfiltration applications.
It is an object of this invention to provide novel microporous fluorocarbon membranes.
It is also an object of this invention to provide novel laminated fluorocarbon membranes useful as the filtration component of a microfiltration cartridge.
It is also an object of this invention to provide a novel microfiltration cartridge.
Another object of this invention is to provide novel microporous laminated polyfluorocarbon membranes and associated cartridge structure to form a microporous polyfluorocarbon filter cartridge with improved strength and flux for microporous filtration.